Resource allocation method for backhaul link and access link in a wireless communication system including relay

ABSTRACT

A resource allocation method of a relay station in a wireless communication system employing the relay station is provided. The method includes: receiving information on a resource allocation pattern for an access link and a backhaul link of a first frequency band; and determining a resource allocation pattern for an access link and a backhaul link of a second frequency band on the basis of the resource allocation pattern of the first frequency band, wherein the first frequency band is any one of an uplink frequency band and a downlink frequency band, and the second frequency band is a remaining one of the uplink frequency band and the downlink frequency band.

TECHNICAL FIELD

The present invention relates to wireless communications, and moreparticularly, to a resource allocation method for a backhaul link and anaccess link in a wireless communication system having a relay station.

BACKGROUND ART

Wireless communication systems are widely spread all over the world toprovide various types of communication services such as voice or data.In general, the wireless communication system is a multiple accesssystem capable of supporting communication with multi-users by sharingavailable radio resources. Examples of the radio resource include time,frequency, code, transmit power, etc. Examples of the multiple accesssystem include a time division multiple access (TDMA) system, a codedivision multiple access (CDMA) system, a frequency division multipleaccess (FDMA) system, an orthogonal frequency division multiple access(OFDMA) system, a single carrier frequency division multiple access(SC-FDMA) system, etc. The radio resource is a time in the TDMA system,a frequency in the FDMA system, a code in the CDMA system, and asubcarrier and a time in the OFDMA system. A wireless communicationsystem is a system supporting bidirectional communication. Thebidirectional communication can be performed by using a time divisionduplex (TDD) mode, a frequency division duplex (FDD) mode, ahalf-frequency division duplex (H-FDD) mode, etc. The TDD mode uses atime resource to identify uplink transmission and downlink transmission.The FDD mode uses a frequency resource to identify uplink transmissionand downlink transmission. The H-FDD mode uses a combination of a timeresource and a frequency resource to identify uplink transmission anddownlink transmission.

The wireless communication system includes a base station (BS) providinga service to a specific region (i.e., a cell). According to acharacteristic of a wireless transmission technology, changes in awireless environment have an effect on quality of signal transmitted. Inparticular, due to various factors in a surrounding environment, such asscatters, movement of a mobile station (MS), etc., a wireless channelchanges over time. In addition, there is a restriction in terms ofdistance since reception power is rapidly decreased in proportion to adistance between wireless communication entities. Therefore, in general,the MS can communicate with the BS when the MS is located within thecoverage of the BS. As such, due to several factors such as thescatters, a movement speed of the MS, a distance between transmissionand reception, etc., a maximum transfer rate, a throughput of anintra-cell user, and a throughput of a whole cell are decreased betweenthe BS and the MS. For example, when the MS is located in a cellboundary or when an obstacle such as a building exists between the MSand the BS, communication quality between the MS and the BS may not besatisfactory.

As an effort to overcome the aforementioned problem, several techniquesare introduced to compensate for deterioration of signals transmittedbetween the BS and the MS, thereby obtaining a maximum transfer rate,throughput improvement, coverage expansion, etc. For this purpose, awireless communication system may employ a relay station (RS). Forexample, a relay technique is one of main techniques for long termevolution (LTE)-advanced which is one of promising candidates ofinternational mobile telecommunication (IMT)-advanced, i.e., a mobilecommunication system of a next generation (post-3^(rd) generation). TheRS is an apparatus for relaying a signal between the BS and the MS, andcan extend the coverage of the BS and improve a cell throughput.

SUMMARY [Technical Problem]

The present invention provides a frame structure supporting a relaystation. More particularly, the present invention provides a method ofproperly allocating resources for a backhaul link between a base stationand a relay station and an access link between the relay station and amobile station.

[Technical Solution]

According to an aspect of the present invention, a resource allocationmethod in a wireless communication system employing a relay station isprovided. The method includes: receiving information on a resourceallocation pattern for an access link and a backhaul link of a firstfrequency band; and determining a resource allocation pattern for anaccess link and a backhaul link of a second frequency band on the basisof the resource allocation pattern of the first frequency band, whereinthe first frequency band is any one of an uplink frequency band and adownlink frequency band, and the second frequency band is any one of theuplink frequency band and the downlink frequency band.

In the aforementioned aspect of the present invention, at least one pairof subframes may be allocated to a subframe for the access link in everyframe which is configured with 10 subframes in the downlink frequencyband.

In addition, the pair of subframes may be two subframes spaced apart by5 subframes.

In addition, the resource allocation pattern of the first frequency bandand the resource allocation pattern of the second frequency band may bedetermined in every frame configured with 10 subframes.

In addition, the resource allocation pattern of the first frequency bandand the resource allocation pattern of the second frequency band may bedetermined in every 40 subframes corresponding to the minimum commonmultiple of the maximum number of hybrid automatic request repeat (HARQ)channels and the number of subframes constituting one frame.

In addition, the method may further include re-determining anacknowledgment/not-acknowledgement (ACK/NACK) feedback time for datatransmission on the basis of the resource allocation pattern of thefirst frequency band and the resource allocation pattern of the secondfrequency band.

In addition, the method may further include reporting information on theACK/NACK feedback time to a mobile station.

According to another aspect of the present invention, a resourceallocation method of a base station in a wireless communication systememploying a relay station is provided. The method includes: determininginformation on a resource allocation pattern for an access link and abackhaul link of a first frequency band; and determining a resourceallocation pattern for an access link and a backhaul link of a secondfrequency band on the basis of the resource allocation pattern of thefirst frequency band, wherein the first frequency band is any one of anuplink frequency band and a downlink frequency band, and the secondfrequency band is any one of the uplink frequency band and the downlinkfrequency band.

In the aforementioned aspect of the present invention, the method mayfurther include transmitting information on the resource allocationpattern of the first frequency band and information on the resourceallocation pattern of the second frequency band to the relay station.

According to another aspect of the present invention, a relay station isprovided. The relay station includes: a processor; and a radio frequency(RF) unit, coupled to the processor, for transmitting and receiving aradio signal, wherein the processor receives information on a resourceallocation pattern for an access link and a backhaul link of a firstfrequency band, and determines a resource allocation pattern for anaccess link and a backhaul link of a second frequency band on the basisof the resource allocation pattern of the first frequency band, andwherein the first frequency band is any one of an uplink frequency bandand a downlink frequency band, and the second frequency band is any oneof the uplink frequency band and the downlink frequency band.

[Advantageous Effects]

According to the present invention, a frame structure supporting a relaystation can be provided. In particular, a resource allocation method fora backhaul link and an access link can be provided. In addition, amethod of reducing an error at an acknowledgement(ACK)/not-acknowledgement (NACK) transmission time can be provided in aprocess of performing hybrid automatic request repeat (HARQ) on thebasis of the frame structure supporting the relay station.

DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing a wireless communication system employing arelay station.

FIG. 2 shows a link configuration among a base station, a relay station,and a mobile station.

FIG. 3 shows a transmission/reception function from the perspective ofan apparatus.

FIG. 4 shows an example of a radio frame structure.

FIG. 5 shows an example of a downlink subframe structure.

FIG. 6 and FIG. 7 show an example of an operation of a relay station.

FIG. 8 shows a frame structure of a downlink frequency band according toan embodiment of the present invention.

FIG. 9 shows a period of determining a resource allocation patternaccording to an embodiment of the present invention.

FIG. 10 shows a period of determining a resource allocation patternaccording to another embodiment of the present invention.

FIG. 11 is a flowchart showing a signaling method of a resourceallocation pattern according to an embodiment of the present invention.

FIG. 12 is a flowchart showing a signaling method of a resourceallocation pattern according to another embodiment of the presentinvention.

FIG. 13 shows a frame structure according to an embodiment of thepresent invention.

FIG. 14 is a flowchart showing a method of determining a resourceallocation pattern according to an embodiment of the present invention.

FIG. 15 is a flowchart showing a method of determining a resourceallocation pattern according to another embodiment of the presentinvention.

FIG. 16 shows an example of performing uplink hybrid automatic repeatrequest (HARQ) by using a method of determining a resource allocationpattern according to an embodiment of the present invention.

FIG. 17 shows another example of performing HARQ by using a method ofdetermining a resource allocation pattern according to an embodiment ofthe present invention.

FIG. 18 and FIG. 19 show a resource allocation pattern allocated basedon a criterion 1 to a criterion 6.

FIG. 20 shows a resource allocation pattern according to anotherembodiment of the present invention.

FIG. 21 is a flowchart showing a resource allocation method according toan embodiment of the present invention.

FIG. 22 is a flowchart showing a resource allocation method according toanther embodiment of the present invention.

FIG. 23 is a block diagram showing a wireless communication systememploying a relay station according to an embodiment of the presentinvention.

MODE FOR INVENTION

The technique described below can be used in various wireless accesstechnologies such as code division multiple access (CDMA), frequencydivision multiple access (FDMA), time division multiple access (TDMA),orthogonal frequency division multiple access (OFDMA), single carrierfrequency division multiple access (SC-FDMA), etc. The CDMA may beimplemented with a radio technology such as universal terrestrial radioaccess (UTRA) or CDMA2000. The TDMA may be implemented with a radiotechnology such as global system for mobile communications (GSM)/generalpacket radio service (GPRS)/enhanced data rates for GSM evolution(EDGE). The OFDMA may be implemented with a radio technology such asinstitute of electrical and electronics engineers (IEEE) 802.11 (Wi-Fi),IEEE 802.16 (WiMAX), IEEE 802-20, evolved-UTRA (E-UTRA) etc. The UTRA isa part of a universal mobile telecommunication system (UMTS). 3rdgeneration partnership project (3GPP) long term evolution (LTE) is apart of an evolved-UMTS (E-UMTS) using the E-UTRA. The 3GPP LTE employsthe OFDMA in a downlink and employs the SC-FDMA in an uplink.

For clarity of explanation, the following description will focus on the3GPP LTE-advanced. However, the technical features of the presentinvention are not limited thereto. 3GPP LTE-advanced (hereinafter,LTE-A) is an evolution of 3GPP LTE (hereinafter, LTE).

FIG. 1 is a diagram showing a wireless communication system employing arelay station. The wireless communication system can be widely deployedto provide a variety of communication services, such as voices, packetdata, etc.

Referring to FIG. 1, the wireless communication system includes mobilestations (MSs) 10, 11, 12, and 13, a base station (BS) 20, and relaystations (RSs) 30 and 31. Each of the MSs 10, 11, 12, and 13 may befixed or mobile, and may be referred to as another terminology, such asa user equipment (UE), a user terminal (UT), a subscriber station (SS),a wireless device, etc. The BS 20 is generally a fixed station thatcommunicates with the MSs 10, 11, 12, and 13 and may be referred to asanother terminology, such as a node-B, a base transceiver system (BTS),an access point, etc. One or more cells may exist in the coverage of oneBS 20. The RSs 30 and 31 are provided for coverage extension or datatransfer rate improvement resulted from a diversity effect, and arelocated between the MS and the BS. The RS may be referred to as anotherterminology, such as a repeater, a relay, a relay node (RN), etc. Thatis, the MSs 10 and 11 located inside the coverage of the BS 20 candirectly communicate with the BS 20, and the MSs 12 and 13 locatedoutside the coverage of the BS 20 communicate with the BS 20 via the RSs30 and 31. Alternatively, for the data transfer rate improvementresulted from the diversity effect, even the MSs 10 and 11 locatedinside the coverage of the BS 20 may communicate with the BS 20 via theRSs 30 and 31.

The RS can be classified into several types as follows.

TABLE 1 Pico/ function L1 Relay L2 Relay L3 Relay Femto Cell RF functionX X X X Coder/Decoder and CRC — X X X HARQ — X X X Multiplex &Demultiplex of MAC SDU — X X X Priority(QoS) handling — X X X Scheduling— X X X Outer ARQ — (X) X X (Re)-Segmentation and concatenation — (X) XX Header compression(ROHC) — — — X Reordering of lower layer SDUs — — —X In-sequence delivery of upper layer PDUs — — — Duplicate detection oflower layer SDUs — — — X Ciphering — — — X System information broadcast— — X X RRC Connection set-up and maintenance — — X X Radio Bearersset-up and maintenance — — — X Mobility function — — X MBMS servicescontrol — — — X Paging — — — X QoS management — — (X) X UE measurementreporting and control the — — (X) X reporting NAS signalling handling —— — X

In Table 1, ‘X’ denotes that a corresponding function is supported, ‘-’denotes that the corresponding function is not supported, and ‘(X)’denotes that the corresponding function may be supported or notsupported. L1 relay, L2 relay, and L3 relay classified in Table 1 arefor exemplary purposes only. The L1, L2, and L3 relays are classifiedherein according to a general feature, and thus other terms may also beused. By reference, Table 1 also shows a function of a femto cell or apico cell. It is assumed that the femto cell or the pico cell supportsall functions shown in Table 1. The L1 relay is a relay having someadditional functions together with amplify and forward (AF). The L1relay amplifies a signal received from a BS or an MS and then deliversthe signal to the MS or the BS. The L2 relay is a relay having ascheduling function together with decoding and forward (DF). The L2relay restores information by performing demodulation, decoding, and thelike on a signal received from the BS or the MS, and then generates asignal by performing coding, modulation, and the like and delivers thesignal to the MS or the BS. The L3 relay is a relay having a patternsimilar to one cell, and supports call access, release, and mobilityfunctions together with the functions of the L2 relay. The L3 relay, thefemto cell, and the pico cell have an ability to change partially orentirely a frame structure. That is, they are RSs capable of controllinga relay cell. On the other hand, the L1 relay and the L2 relay do nothave the ability to change partially or entirely the frame structure.That is, they are RSs not capable of controlling the relay cell.Therefore, the L1 relay and the L2 relay are used to relay only data,and the BS directly transmits a control channel to the MS. Unlessotherwise specified, the L1 relay, the L2 relay, the L3 relay, the picocell, and the femto cell are collectively referred to as an RS in thepresent invention.

The RS can transmit or receive data by using a radio resource. The radioresource that can be used by the RS includes a time resource, afrequency resource, a space resource, etc. The time resource isexpressed by a subframe, a symbol, a slot, etc. The frequency resourceis expressed by a subcarrier, a resource block, a component carrier,etc. The space resource is expressed by spatial multiplexing, anantenna, etc. The radio resource can be dedicatedly used between a BSand an RS or between the RS and an MS, or can be shared between them.

FIG. 2 shows a link configuration among a BS, an RS, and an MS.

Referring to FIG. 2, a link between the BS and the RS is a backhaullink, and a link between the RS and the MS is an access link. Thebackhaul link between the BS and the RS can operate in a frequencydivision duplex (FDD) mode or a time division duplex (TDD) mode.Likewise, the access link between the RS and the MS can operate in theFDD mode or the TDD mode. In the FDD mode, uplink and downlink aredivided by a frequency resource. In the TDD mode, uplink and downlinkare divided by a time resource. In the FDD mode, a backhaul downlink iswhen transmission is performed on the backhaul link by using a downlinkfrequency band, and a backhaul uplink is when transmission is performedon the backhaul link by using an uplink frequency band. In addition, anaccess downlink is when transmission is performed on the access link byusing the downlink frequency band, and an access uplink is whentransmission is performed on the access link by using the uplinkfrequency band. In the TDD mode, a backhaul downlink is whentransmission is performed on the backhaul link by using a downlinksubframe, and a backhaul uplink is when transmission is performed on thebackhaul link by using an uplink subframe. In addition, an accessdownlink is when transmission is performed on the access link by usingthe downlink subframe, and an access uplink is when transmission isperformed on the access link by using the uplink subframe. A radioresource allocated for the backhaul link is referred to as a relay zone,and a radio resource allocated for the access link is referred to as anaccess zone.

Table 2 below summarizes transmission/reception functions of the BS, theRS, and the MS.

TABLE 2 Function BS RS MS downlink transmission V V downlink reception VV uplink transmission V V uplink reception V V

Referring to Table 2, the BS can perform transmission by using adownlink resource, and can perform reception by using an uplinkresource. The MS can perform reception by using the downlink resource,and can perform transmission by using the uplink resource. On the otherhand, the RS can perform transmission or reception by using the downlinkresource, and can perform transmission and reception by using the uplinkresource.

FIG. 3 shows a transmission/reception function from the perspective ofan apparatus. For convenience of explanation, the present inventionassumes that an RS operates in an FDD mode. However, this is forexemplary purposes only, and thus the RS can also operate in a TDD modeor a half-FDD (H-FDD) mode. In addition, it is also assumed that the RSuses an orthogonal frequency division multiplex access (OFDMA) scheme indownlink transmission, and uses a discrete Fouriertransform-spread-OFDMA (DFT-s-OFDMA) scheme in uplink transmission. TheDFT-s-OFDMA scheme can also be referred to as a single carrier-frequencydivision multiplex access (SC-FDMA) scheme, and both terms are usedtogether.

Referring to FIG. 3, the RS may include two reception modules forreceiving a signal from a BS or an MS. Further, the RS may include twotransmission modules for transmitting a signal to the BS or the MS. Foreasy understanding, the transmission and reception modules aresimplified to include only a duplexer and a signal processing module.The duplexer is either a unidirectional duplexer or a bidirectionalduplexer according to a function. In case of the unidirectionalduplexer, as shown in FIG. 3, each duplexer is coupled to only any oneof the transmission module and the reception module. On the other hand,in case of the bidirectional duplexer, each duplexer can be coupled toboth of the transmission module and the reception module.

In general, the RS is prevented from performing transmission andreception operations simultaneously, and thus the transmission moduleand the reception module can process signals at different times. On theother hand, the RS is allowed to receive signals simultaneously from theBS and the MS or transmit signals simultaneously to the BS and the MS.Therefore, parallel signal processing can be performed at the same timeor similar times between the transmission modules or between thereception modules.

In the signal processing, a downlink signal received from the BS isprocessed by fast Fourier transform (FFT) and an OFDMA basebandreception process. Similarly, a downlink signal to be transmitted to theMS is processed by an OFDMA baseband transmission process and inversefast Fourier transform (IFFT). Specifically, the OFDMA basebandtransmission process may include a serial/parallel conversion module anda subcarrier mapping module. That is, the OFDMA baseband transmissionprocess converts a high-speed serial data symbol into a slow paralleldata symbol, and then maps the resultant symbol to a subcarrier. TheOFDMA baseband reception process processes a signal in the oppositemanner as the OFDMA baseband transmission process.

Meanwhile, an uplink signal received from the MS is processed by FFT anda DFT-s-OFDMA baseband reception process. Similarly, an uplink signal tobe transmitted to the BS is processed by a DFT-s-OFDMA basebandtransmission process and IFFT. Specifically, the DFT-s-OFDMA basebandtransmission process may include a serial/parallel conversion module, aDFT module, and a subcarrier mapping module. The DFT-s-OFDMA basebandtransmission process differs from the OFDMA baseband transmissionprocess in that a data symbol can be spread in a frequency domain byusing DFT before IFFT in order to decrease a peak-to-average power ratio(PAPR) of a transmission symbol. A data symbol subjected to the DFTmodule can be either contiguously or discontinuously mapped to asubcarrier. The DFT-s-OFDMA baseband reception process processes asignal in the opposite manner as the DFT-s-OFDMA baseband transmissionprocess.

FIG. 4 shows an example of a radio frame structure. Herein, a radioframe uses a normal cyclic prefix (CP).

Referring to FIG. 4, the radio frame includes 10 subframes. One subframemay include two slots. One slot may include a plurality of OFDM symbolsor SC-FDMA symbols in a time domain. The number of OFDM symbols orSC-FDMA symbols included in one slot may be determined variouslyaccording to a CP configuration. In a radio frame using a normal CPsize, one slot may include 7 OFDM symbols or SC-FDMA symbols. When anOFDM symbol or an SC-FDMA symbol has a length of 2048 Ts in a radioframe having a length of 10 ms, the normal CP size may be 144 Ts(Ts=1/(15000*2048)sec).

In case of a downlink radio frame, a primary synchronization channel(P-SCH) is located in a last OFDM symbol of a 0^(th) slot and a 10^(th)slot. The same primary synchronization signal (PSS) is transmittedthrough 2 P-SCHs. The P-SCH is used to obtain time domainsynchronization such as OFDM symbol synchronization, slotsynchronization and/or frequency domain synchronization. A Zadoff-Chu(ZC) sequence can be used as the PSS. At least one PSS exists in awireless communication system.

A secondary synchronization channel (S-SCH) is located in an immediatelyprevious OFDM symbol from the last OFDM symbol of the 0^(th) slot andthe 10^(th) slot. The S-SCH and the P-SCH can be located in contiguousOFDM symbols. Different secondary synchronization signals (SSSs) aretransmitted through two S-SCHs. The S-SCH is used to obtain framesynchronization and/or CP configuration of a cell, i.e., usageinformation of a normal CP or an extended CP. One S-SCH uses two SSSs.An m-sequence may be used as the SSS. That is, one S-SCH includes twom-sequences. For example, if one S-SCH includes 63 subcarriers, twom-sequences each having a length of 31 are mapped to S-SCH.

The P-SCH and the S-SCH are used to obtain physical-layer cellidentities (IDs). The physical-layer cell ID can be expressed by 168physical-layer ID groups and 3 physical-layer IDs belonging to eachphysical-layer ID group. That is, a total number of physical-layer cellIDs is 504, and the physical-layer cell IDs are expressed by aphysical-layer ID group in the range of 0 to 167 and physical-layer IDsincluded in each physical-layer cell ID and having a range of 0 to 2.The P-SCH may use 3 ZC sequence root indices indicating thephysical-layer IDs. The S-SCH may use 168 m-sequence indices indicatingthe physical-layer cell ID groups.

A physical-broadcast channel (P-BCH) is located in a 0^(th) subframe inthe radio frame. The P-BCH starts from a 3r^(d) OFDM symbol (startingfrom a 0^(th) OFDM symbol) of the 0^(th) subframe and occupies 4 OFDMsymbols excluding the P-SCH and the S-SCH. The P-BCH is used to obtainbasic system configuration information of a corresponding BS. The P-BCHmay have a period of 40 ms.

The radio frame structure of FIG. 4 is for exemplary purposes only, andthus the number of subframes included in the radio frame or the numberof slots included in the subframe may change variously.

FIG. 5 shows an example of a downlink subframe structure. A subframeincludes two slots in a time domain. Up to three preceding OFDM symbolsof a 1^(st) slot in the subframe correspond to a control region to beallocated with physical channels. The remaining OFDM symbols correspondto a data region to be allocated with a physical downlink shared channel(PDSCH).

Examples of downlink control channels used in 3^(rd) generationpartnership project (3GPP) long term evolution (LTE) include a physicalcontrol format indicator channel (PCFICH), a physical downlink controlchannel (PDCCH), a physical hybrid-ARQ indicator channel (PHICH), etc.The PCFICH transmitted in a first OFDM symbol of a subframe carriesinformation regarding the number of OFDM symbols (i.e., a size of thecontrol region) used for transmission of control channels in thesubframe. Control information transmitted through the PDCCH is referredto as downlink control information (DCI). The DCI indicates uplinkresource allocation information, downlink resource allocationinformation, an uplink transmit power control command for any UE groups,etc. The PHICH carries an acknowledgement (ACK)/not-acknowledgement(NACK) signal for an uplink hybrid automatic repeat request (HARQ). Thatis, an ACK/NACK signal for uplink data transmitted by a UE istransmitted through the PHICH.

FIG. 6 and FIG. 7 show an example of an operation of an RS. Hereinafter,unless otherwise specified, an MS connected to the RS is simply referredto as an MS. FIG. 6 shows an example of downlink transmission using adownlink frequency band f_(D). FIG. 7 shows an example of uplinktransmission using an uplink frequency band f_(U). It is assumed thatthe downlink frequency band f_(D) and the uplink frequency band f_(U)are different frequency bands.

Referring to FIG. 6, when an RS receives a signal from a BS through abackhaul link, the RS cannot transmit the signal to an MS through anaccess link. When the RS transmits a signal to the MS through the accesslink, the RS cannot receive the signal from the BS through the backhaullink.

Referring to FIG. 7, when an RS receives a signal from an MS through anaccess link, the RS cannot transmit the signal to a BS through abackhaul link. When the RS transmits a signal to the BS through thebackhaul link, the RS cannot receive the signal from the MS through theaccess link.

As shown in FIG. 6 and FIG. 7, the RS cannot simultaneously transmit andreceive signals by using the same frequency band. Therefore, a framestructure of the RS needs to be defined for each of the downlinkfrequency band and the uplink frequency band. In particular, there is aneed to allocate a resource for the backhaul link and the access link.

Hereinafter, a criterion for allocating a resource for a backhaul linkand an access link in a downlink frequency band and an uplink frequencyband will be proposed according to an embodiment of the presentinvention.

Criterion 1

The backhaul link and the access link follow a time divisionmultiplexing (TDM) scheme in each of the downlink frequency band and theuplink frequency band. In this case, the backhaul link and the accesslink can be allocated in a unit of one subframe or a multiple of onesubframe. For example, one subframe is 1 ms.

Criterion 2

At least one pair of subframes in one frame must be allocated for theaccess link in the downlink frequency band. For example, one pair ofsubframes may include a k^(th) subframe and a (k+5)^(th) subframe.

Criterion 3

A resource allocation pattern for the backhaul link and the access linkis determined in every specific period.

Criterion 4

The resource allocation pattern for the backhaul link and the accesslink is determined by the BS, and are signaled to each RS.

Criterion 5

When switching is made in a frame from the backhaul link to the accesslink or from the access link to the backhaul link, some regions can bedetermined as an idle time for TTG/RTG.

Criterion 6

When the BS first determines a resource allocation pattern for any oneof the downlink frequency band and the uplink frequency band, a resourceallocation pattern of the remaining one frequency band is determinedbased on the first determined resource allocation pattern.

Hereinafter, a method of determining a resource allocation pattern for abackhaul link and an access link according to the criterion 2 to thecriterion 6 will be described in detail for example in reference to FIG.8 to FIG. 15.

FIG. 8 shows a frame structure of a downlink frequency band according toan embodiment of the present invention (criterion 2).

Referring to FIG. 8, a frame consists of 10 subframes. Among subframesconstituting the frame, a subframe #0 and a subframe #5 can constituteone pair. Likewise, (a subframe #1 and a subframe #6), (a subframe #2and a subframe #7), (a subframe #3 and a subframe #8), and (a subframe#4 and a subframe #9) each constitute one pair. Among them, at least onepair of subframes must be allocated to a subframe for the access link.This is to transmit a control signal to MSs belonging to the coverage ofan RS. Examples of the control signal include a synchronization signal,a broadcast signal, a paging signal, etc. For example, in an FDD frame,the subframe #0 and the subframe #5 are subframes in which thesynchronization signal and/or the broadcast signal are transmitted, andthe subframe #0, the subframe #4, the subframe #5, and the subframe #9are subframes in which the paging signal is transmitted. Therefore, (thesubframe #0 and the subframe #5) and (the subframe #4 and the subframe#9) can be allocated for the access link. Likewise, in a TDD frame, thesubframe #0 and the subframe #5 are subframes in which thesynchronization signal and/or the broadcast signal are transmitted, andthe subframe #0, the subframe #1, the subframe #5, and the subframe #6are subframes in which the paging signal is transmitted. Therefore, (thesubframe #0 and the subframe #5) and (the subframe #1 and the subframe#6) can be allocated for the access link. Accordingly, in the FDD frame,the subframe #0, the subframe #4, the subframe #5, and the subframe #9must be allocated for the access link, and cannot be allocated for thebackhaul link, and in the TDD frame, the subframe #0, the subframe #1,the subframe #5, and the subframe #6 must be allocated for the accesslink, and cannot be allocated to the backhaul link.

FIG. 9 shows a period of determining a resource allocation patternaccording to an embodiment of the present invention (criterion 3).

Referring to FIG. 9, the resource allocation pattern can be determinedwith a period of one frame (i.e., 10 ms). In this case, since one frameconsists of 10 subframes, the resource allocation pattern can beexpressed with 10 bits. Alternatively, the resource allocation patterncan be expressed with 6 bits except for 4 subframes in which asynchronization signal, a broadcast signal, and a paging signal must betransmitted. That is, as shown in FIG. 9, when a subframe #0, a subframe#4, a subframe #5, and a subframe #9 are determined as subframes fortransmitting the synchronization signal, the broadcast signal, and thepaging signal, and when a subframe #1, a subframe #2, and a subframe #8are allocated for the backhaul link and a subframe #3, a subframe #6,and a subframe #7 are allocated for the access link, the resourceallocation pattern can be expressed by “001110”. Herein, “0” and “1” canbe interpreted oppositely.

FIG. 10 shows a period of determining a resource allocation patternaccording to another embodiment of the present invention (criterion 3).

Referring to FIG. 10, the resource allocation pattern can be determinedwith a period corresponding to the least common multiple of the numberof hybrid automatic request repeat (HARQ) channels and the number ofsubframes included in one frame. That is, according to an LTE standard,since a time required from initial data transmission to retransmissionis 8 ms in the process of performing HARQ, the number of HARQ channelscan be 8. Therefore, the resource allocation pattern can be determinedin a unit of 40 ms which is the least common multiple of 8 and 10. Inthis case, the resource allocation pattern can be expressed by 40 bits.Alternatively, the resource allocation pattern can be expressed by 24bits except for subframes in which a synchronization signal, a broadcastsignal, and a paging signal must be transmitted.

FIG. 11 is a flowchart showing a signaling method of a resourceallocation pattern according to an embodiment of the present invention(criterion 4). It is assumed that a plurality of RSs exist for one BS.

Referring to FIG. 11, the BS determines the resource allocation patternfor the backhaul link and the access link for each RS (step S100). TheBS transmits information on a resource allocation pattern determined fora first RS to the first RS (step S110), and transmits information on aresource allocation pattern for a second RS to the second RS (stepS120). In this case, the BS can unicast information on the resourceallocation pattern to each RS through a PDSCH.

FIG. 12 is a flowchart showing a signaling method of a resourceallocation pattern according to another embodiment of the presentinvention (criterion 4). It is assumed herein that a plurality of RSsexist for one BS.

Referring to FIG. 12, the BS determines a resource allocation patternfor the backhaul link and the access link for all RSs (step S200). TheBS broadcasts information on the resource allocation pattern determinedin step 200 to all RSs (step S210). In this case, the BS can broadcastinformation on the resource allocation pattern to the RS through a BCH.In addition thereto, the BS can unicast information on the same resourceallocation pattern to each RS.

Referring to FIG. 11 and FIG. 12, the BS and the RS can share a tablefor a predetermined resource allocation pattern, and the BS can signalan index for a specific pattern to the RS.

FIG. 13 shows a frame structure according to an embodiment of thepresent invention (criterion 5).

Referring to FIG. 13, when switching is made from the backhaul link tothe access link or from the access link to the backhaul link, someregions immediately before the switching is achieved are configured asan idle time. For example, the idle time may be one OFDMA symbol. Asubframe #1 and a subframe #6 in which the switching is made from theaccess link to the backhaul link and a subframe #3 and a subframe #9 inwhich the switching is made from the backhaul link to the access linkare subframes including the idle time, and such subframes can beregarded as irregular subframes.

FIG. 14 is a flowchart showing a method of determining a resourceallocation pattern according to an embodiment of the present invention(criterion 6).

Referring to FIG. 14, a BS determines a resource allocation pattern fora backhaul link and an access link for a downlink frequency band (stepS300). On the basis of the resource allocation pattern of the downlinkfrequency band determined in step S300, the BS determines a resourceallocation pattern for a backhaul link and an access link for an uplinkfrequency band (step S310). The downlink frequency band' resourceallocation pattern and the uplink frequency band' resource allocationpattern determined in step S300 and step S310 are transmitted to an RS(step S320).

FIG. 15 is a flowchart showing a method of determining a resourceallocation pattern according to another embodiment of the presentinvention.

Referring to FIG. 15, a BS determines a resource allocation pattern fora backhaul link and an access link for a downlink frequency band (stepS400). The downlink frequency band's resource allocation patterndetermined in step S400 is transmitted to an RS (step S410). On thebasis of the downlink frequency band's resource allocation patternreceived in step S410, the RS determines a resource allocation patternof an uplink frequency band. (step S420)

In FIG. 14 and FIG. 15, the resource allocation pattern of the downlinkfrequency band is first determined, and then the resource allocationpattern of the uplink frequency band is determined on the basis of thefirst determined resource allocation pattern. However, this is forexemplary purposes only, and thus the resource allocation pattern of theuplink frequency band can be first determined and then the resourceallocation pattern of the downlink frequency band can be determined onthe basis of the first determined resource allocation pattern.

FIG. 16 shows an example of performing HARQ by using a method ofdetermining a resource allocation pattern according to an embodiment ofthe present invention. When performing the HARQ, a transmitter Txtransmits data by using a subframe, and receives a feedback ofacknowledgment (ACK)/not-acknowledgement (NACK) on the subframe from areceiver Rx, and then retransmits the data. A round trip time (RRT) isdefined as a time from data transmission of the transmitter to dataretransmission. According to an LTE specification, the receiver feedsback ACK/NACK after 4 subframes elapse from when the transmittertransmits data.

Referring to FIG. 16, a subframe #k of a downlink frequency band isdetermined as a subframe for an access link. Therefore, an RS cantransmit downlink data to an MS through the subframe #k of the downlinkfrequency band. Upon receiving the downlink data from the RS, the MSfeeds back ACK/NACK for the downlink data after a specific time elapsesfrom when the downlink data is transmitted. For example, the MS feedsback ACK/NACK after 4 subframes elapse from when the downlink data istransmitted. Therefore, a subframe #k+4 of an uplink frequency band isdetermined as the subframe for the access link, and the MS feeds backACK/NACK to the RS through the subframe #k+4.

FIG. 17 shows another example of performing HARQ by using a method ofdetermining a resource allocation pattern according to an embodiment ofthe present invention.

Referring to FIG. 17, a subframe #k of an uplink frequency band isdetermined as a subframe for an access link. Therefore, an MS cantransmit uplink data to an RS through the subframe of the uplinkfrequency band. Upon receiving the uplink data from the MS, the RS feedsback ACK/NACK for the uplink data through a PHICH after a specific time(e.g., 4 subframes) elapses from when the uplink data is transmitted.Therefore, a subframe #k+4 of a downlink frequency band is determined asthe subframe for the access link, and the RS feeds back ACK/NACK to theMS through the subframe #k+4 of the downlink frequency band.

As shown in FIG. 16 and FIG. 17, when considering HARQ, if a k^(th)subframe of the downlink frequency band is determined as the subframefor the access link, a (k+n)^(th) subframe (e.g., a (k+4)^(th) subframe)of the uplink frequency band is also determined as the subframe for theaccess link. Alternatively, when a k^(th) subframe of the uplinkfrequency band is determined as the subframe for the access link, a(k+n)^(th) subframe of the downlink frequency band is also determined asthe subframe for the access link. Likewise, when a k^(th) subframe ofthe downlink frequency band is determines as a subframe for a backhaullink, a (k+n)^(th) subframe (e.g., a (k+4)^(th) subframe) of the uplinkfrequency band is also determined as the subframe from the backhaullink. Alternatively, when a k^(th) subframe of the uplink frequency bandis determined as a subframe for the backhaul link, a (k+n)^(th) subframeof the downlink frequency band is also determined as the subframe forthe backhaul link.

Hereinafter, a problem that may occur when a resource allocation patternis determined based on the criterion 1 to the criterion 6 and a methodof solving the problem will be described.

First, when a resource allocation pattern for a backhaul link and anaccess link is determined with a period of one frame (i.e., 10 ms),there may be a case where the criterion 6 cannot be satisfied at leastonce.

FIG. 18 and FIG. 19 show a resource allocation pattern allocated basedon the criterion 1 to the criterion 6.

Referring to FIG. 18, a resource allocation pattern for a downlinkfrequency band is first determined, and a resource allocation pattern ofan uplink frequency band is determined on the basis of the firstdetermined resource allocation pattern. Herein, when an RS transmitsdownlink data to an MS through a subframe for an access link of thedownlink frequency band, each MS can feed back ACK/NACK to the RSthrough a subframe for an access link of the uplink frequency band after4 subframes elapse from when the downlink data is transmitted. Likewise,when the RS receives downlink data from a BS through a subframe for abackhaul link of the downlink frequency band, the RS can feed backACK/NACK to the BS through a subframe for a backhaul link of the uplinkfrequency band after 4 subframes elapse from when the downlink data istransmitted.

However, when the resource allocation pattern of the uplink frequencyband is determined based on the resource allocation pattern of thedownlink frequency band as shown in FIG. 18, an error may occur in somesubframes. For example, a subframe #0 of the uplink frequency band isdetermined as the subframe for the access link on the basis of arelation with the downlink frequency band. When the MS transmits uplinkdata to the RS through the subframe #0 of the uplink frequency band, theRS must feed back ACK/NACK to the MS through a subframe #4 of thedownlink frequency band. However, since the subframe #4 of the downlinkfrequency band is determined as the subframe for the backhaul link, theRS cannot feed back ACK/NACK to the MS within a determined time.

Such an error may also occur in an opposite case. Referring to FIG. 19,a resource allocation pattern of the uplink frequency band is firstdetermined, and a resource allocation pattern of the downlink frequencyband is determined on the basis of the first resource allocationpattern. Herein, when an MS transmits uplink data to an RS through asubframe for an access link of the uplink frequency band, the RS canfeed back ACK/NACK to the MS through a subframe for an access link ofthe downlink frequency band after 4 subframes elapse from when theuplink data is transmitted. Likewise, when the RS transmits uplink datato a BS through a subframe for a backhaul link of the uplink frequencyband, the BS can feed back ACK/NACK to the RS through a subframe for thebackhaul link of the downlink frequency band after 4 subframes elapsefrom when the uplink data is transmitted.

However, when the resource allocation pattern of the downlink frequencyband is determined on the basis of the resource allocation pattern ofthe uplink frequency band as shown in FIG. 19, an error may occur insome subframes. For example, a subframe #0 of the downlink frequencyband is determined as the subframe for the access link on the basis of arelation with the uplink frequency band. When the RS transmits downlinkdata to the MS through the subframe #0 of the downlink frequency band,the MS must feed back ACK/NACK to the RS through a subframe #4 of theuplink frequency band. However, the subframe #4 of the uplink frequencyband is determined as the subframe for the backhaul link. Therefore, theRS cannot feed back ACK/NACK to the MS within a determined time. Foranother example, a subframe #2 of the downlink frequency band isdetermined as the subframe for the backhaul link on the basis of arelation with the uplink frequency band. When the BS transmits downlinkdata to the RS through a subframe #2 of the downlink frequency band, theRS must feed back ACK/NACK to the BS through a subframe #6 of the uplinkfrequency band. However, the subframe #6 of the uplink frequency band isdetermined as the subframe for the access link. Therefore, the BS cannotfeed back ACK/NACK to the RS within a determined time.

Next, an error may also occur when the resource allocation pattern forthe backhaul link and the access link is determined with a periodcorresponding to the least common multiple (i.e., 40 ms) of the numberof HARQ channels and the number of subframes in one frame.

FIG. 20 shows a resource allocation pattern according to anotherembodiment of the present invention.

Referring to FIG. 20, the resource allocation pattern is determined witha period of 40 ms. A resource allocation pattern of either a downlinkfrequency band or an uplink frequency band is first determined, and thena resource allocation pattern of the remaining frequency band isdetermined on the basis of the first determined resource allocationpattern. Herein, since a period of the resource allocation pattern is 40ms and an HARQ operation of 8 channels is considered, an erroroccurrence probability of FIG. 18 and FIG. 19 is relatively low.However, since at least one pair of subframes spaced apart by 5subframes must be configured as a subframe for an access link in thedownlink frequency band as in the criterion 2, an error may occur. Forexample, a subframe #0 and a subframe #5 of the downlink frequency bandmust be configured as the access link for transmission of asynchronization signal. Therefore, an error may occur in a resourceallocation pattern of the uplink frequency band corresponding thereto.

Hereinafter, a method of solving the aforementioned problem will bedescribed.

FIG. 21 is a flowchart showing a resource allocation method according toan embodiment of the present invention. Although an HARQ operation in anaccess link between an RS and an MS is described herein for example, thepresent invention can also apply to an HARQ operation in a backhaul linkbetween a BS and the RS.

Referring to FIG. 21, the BS determines a resource allocation pattern ofa downlink frequency band (step S500). The BS transmits information onthe downlink frequency band's resource allocation pattern determined instep 500 to the RS (step S510). The RS determines a resource allocationpattern of an uplink frequency band on the basis of the informationwhich is received in step 510 and which regards to the resourceallocation pattern of the downlink frequency band (step S520). On thebasis of the resource allocation pattern of the downlink frequency bandand the resource allocation pattern of the uplink frequency band, the RSre-determines HARQ timing (step S530). Herein, the HARQ timing may implya duration from an initial data transmission time to an ACK/NACKfeedback time. When the RS receives data from the MS (step S540),ACK/NACK is fed back to the MS in accordance with the HARQ timingdetermined in step S530 (step S550).

For example, in FIG. 18, if the MS transmits uplink data to the RS in asubframe #0 of an uplink frequency band, the RS cannot feed backACK/NACK to the MS in a subframe #4 of a downlink frequency band. Thisis because the subframe #4 of the downlink frequency band is determinedto a subframe for a backhaul link. In this case, the RS can feed backACK/NACK through a PHICH at a time delayed by one subframe from apre-defined time by using the HARQ timing re-determined in step S530.Referring to FIG. 18, ACK/NACK for data transmitted from the MS to theRS in a subframe #1 of the uplink frequency band is determined to betransmitted in a subframe #5 of the downlink frequency band. In thiscase, ACK/NACK for data transmitted in the subframe #0 of the uplinkfrequency band and ACK/NACK for data transmitted in the subframe #1 ofthe uplink frequency band can be transmitted in the subframe #5 of thedownlink frequency band by being multiplexed using at least one of time,frequency, and code.

Herein, the RS may report information on the resource allocation patternof the uplink frequency band, the resource allocation pattern of thedownlink frequency band, and the re-determined HARQ timing to the MS.Accordingly, the MS can know a reception time of ACK/NACK.

FIG. 22 is a flowchart showing a resource allocation method according toanther embodiment of the present invention. Although an HARQ operationin an access link between an RS and an MS is described for example inthe present embodiment, the present invention can also apply to an HARQoperation in a backhaul link between a BS and the MS.

Referring to FIG. 22, the BS determines a resource allocation pattern ofan uplink frequency band (step S600). The BS transmits information onthe uplink frequency band's resource allocation pattern determined instep 600 to the RS (step S610). The RS determines a resource allocationpattern of a downlink frequency band on the basis of the informationwhich is received in step 610 and which regards to the resourceallocation pattern of the uplink frequency band (step S620). On thebasis of the resource allocation pattern of the uplink frequency bandand the resource allocation pattern of the downlink frequency band, theRS re-determines an ACK/NACK transmission time (step S630), and reportsinformation on the ACK/NACK transmission time to the MS (step S640).Herein, the ACK/NACK transmission time may be a subframe allocated forthe access link before or after 4 subframes from a data transmissiontime. The RS can report information on the ACK/NACK transmission time tothe MS through a PDCCH or a medium access control (MAC) protocol dataunit (PDU). Therefore, when the RS transmits downlink data to the MS(step S650), the RS receives ACK/NACK from the MS in accordance with theACK/NACK transmission time determined in step S640 (step S660).

For example, in FIG. 19, if the RS transmits downlink data to the MS ina subframe #0 of a downlink frequency band, the MS cannot feed backACK/NACK to the RS in a subframe #4 of an uplink frequency band. This isbecause the subframe #4 of the uplink frequency band is determined to asubframe for the backhaul link. In this case, the MS can feed backACK/NACK through a PUCCH at a time delayed by one subframe from apre-defined time by using the ACK/NACK transmission time received instep S640.

As such, ACK/NACK for data transmission indicated by a dotted line inFIG. 18 and FIG. 19 can be determined to be fed back through a subframein which ACK/NACK can be transmitted among subframes before or after acorresponding subframe. In addition, in FIG. 20, a subframe #5 of an oddframe or an even frame can be determined as the subframe for thebackhaul link. Accordingly, an MS coupled to the RS in the odd frame orthe even frame may be restricted in receiving a synchronization signalfrom the RS, but an error in an HARQ process can be avoided. Further,when information indicating that the synchronization signal cannot bereceived in a specific frame is signaled to the MS coupled to the RS, aproblem caused by non-transmission of the synchronization signal can beavoided.

FIG. 23 is a block diagram showing a wireless communication systememploying an RS according to an embodiment of the present invention.

Referring to FIG. 23, the wireless communication system includes a BS100, an RS 200, and a UE 300. The BS 100 includes a processor 110 and aradio frequency (RF) unit 120. The RF unit 120 of the BS transmits andreceives a radio signal. The processor 110 determines a resourceallocation pattern of an uplink frequency band and/or a resourceallocation pattern of a downlink frequency band, and transmitsinformation on the resource allocation pattern of the uplink frequencyband and/or the resource allocation pattern of the downlink frequencyband to the RS 200. The RS 200 includes an RF unit 220 and a processor210. The RF unit 220 of the RS 200 transmits and receives a radiosignal. The processor 210 receives information on at least one of theresource allocation pattern of the uplink frequency band and theresource allocation pattern of the downlink frequency band from the BS,and determines the remaining resource allocation patterns on the basisof the information. The UE 300 includes an RF unit 320 and a processor310. The RF unit 320 of the UE 300 transmits and receives a radiosignal. The processor 310 of the UE 300 receives information on theresource allocation pattern of the uplink frequency band and/or theresource allocation pattern of the downlink frequency band from the RS,and communicates with the RS.

The present invention can be implemented with hardware, software, orcombination thereof. In hardware implementation, the present inventioncan be implemented with one of an application specific integratedcircuit (ASIC), a digital signal processor (DSP), a programmable logicdevice (PLD), a field programmable gate array (FPGA), a processor, acontroller, a microprocessor, other electronic units, and combinationthereof, which are designed to perform the aforementioned functions. Insoftware implementation, the present invention can be implemented with amodule for performing the aforementioned functions. Software is storablein a memory unit and executed by the processor. Various means widelyknown to those skilled in the art can be used as the memory unit or theprocessor.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the spirit and scope of theinvention as defined by the appended claims. The exemplary embodimentsshould be considered in descriptive sense only and not for purposes oflimitation. Therefore, the scope of the invention is defined not by thedetailed description of the invention but by the appended claims, andall differences within the scope will be construed as being included inthe present invention.

1-10. (canceled)
 11. A method of communicating in a wirelesscommunication system, performed by a relay station (RS), the methodcomprising: receiving configuration information, from a base station(BS), the configuration information indicating at least one backhaullink subframe which is used for transmission between the BS and the RSwith a fixed period; configuring at least one subframe based on theconfiguration information; and communicating with the BS by using the atleast one configured subframe, wherein the at least one configuredsubframe is selected among the at least one backhaul link subframe withan exception that predefined subframes of a radio frame, the radio frameincluding ten subframes, are excluded.
 12. The method of claim 11,wherein the predefined subframes are a first, a fifth, a sixth and atenth subframe of a radio frame when the BS and the RS are operated infrequency division duplex.
 13. The method of claim 12, furthercomprising; connecting with a UE; and transmitting data to the UE in thefirst, the fifth, the sixth and the tenth subframes.
 14. The method ofclaim 11, wherein the predefined subframes are a first, a second, asixth and a seventh subframe of a radio frame when the BS and the RS areoperated in time division duplex.
 15. The method of claim 14, furthercomprising; connecting with a UE; and transmitting data to the UE in thefirst, the second, the sixth and the seventh subframes.
 16. The methodof claim 11, wherein the fixed period is 8 ms, and a length of eachsubframe is 1 ms.
 17. The method of claim 11, wherein the configurationinformation is received via a higher layer signal.
 18. The method ofclaim 11, wherein the fixed period is related to a hybrid automaticrepeat request(HARQ) round trip time (RTT).
 19. The method of claim 11,wherein the configuration information includes a bitmap which indicatesthe at least one backhaul link subframe.
 20. The method of claim 19,wherein the bitmap comprises 8 bits.
 21. A relay station(RS) comprising:a processor; and a radio frequency (RF) unit, coupled to the processor,for transmitting and receiving a radio signal, wherein the processorreceives configuration information, from a base station (BS), theconfiguration information indicating at least one backhaul link subframewhich is used for transmission between the BS and the RS with a fixedperiod; configures at least one subframe based on the configurationinformation; and communicates with the BS by using the at least oneconfigured subframe, wherein the at least one configured subframe isselected among the at least one backhaul link subframe with an exceptionthat that predefined subframes of a radio frame, the radio frameincluding ten subframes, are excluded.
 22. The RS of claim 21, whereinthe predefined subframes are a first, a fifth, a sixth and a tenthsubframe of a radio frame when the BS and the RS are operated infrequency division duplex.
 23. The RS of claim 22, wherein the processortransmits data to the UE in the first, the fifth, the sixth and thetenth subframes.
 24. The RS of claim 21, wherein the predefinedsubframes are a first, a second, a sixth and a seventh subframe of aradio frame when the BS and the RS are operated in time division duplex.25. The RS of claim 24, wherein the processor transmits data to the UEin the first, the second, the sixth and the seventh subframes.
 26. TheRS of claim 21, wherein the fixed period is 8 ms, and a length of eachsubframe is 1 ms.
 27. The RS of claim 21, wherein the configurationinformation is received via a higher layer signal.
 28. The RS of claim21, wherein the fixed period is related to a hybrid automatic repeatrequest(HARQ) round trip time (RTT).
 29. The RS of claim 21, wherein theconfiguration information includes a bitmap which indicates the at leastone backhaul link subframe.
 30. The RS of claim 29, wherein the bitmapcomprises 8 bits.